1. Field of the Invention
The present invention relates generally to a baseband signal multiplexing circuit provided in a transmitter using a spread spectrum communication system for multiplexing a plurality of baseband signals spread with different spread codes, and more particularly to a method of controlling a transmission level of a code-multiplexed baseband signal.
2. Description of the Related Art
In recent years, attention is focused on a spread spectrum communication system resistant to interference and jamming as a communication system for use in a mobile communication system. In the spread spectrum communication system, a transmission side first spreads a digital signal such as voice and image data using a spread code such as a pseudorandom noise code to convert it into a baseband signal with a broader bandwidth than that of the original digital signal. The baseband signal is modulated with modulating systems such as PSK (Phase Shift Keying), FSK (Frequency Shift Keying) or the like to form a high frequency signal which is then transmitted.
One of the spread spectrum communication systems is a DS (Direct Sequence) system. Spread codes used in the system are allocated for respective communication channels with cross-correlation thereof being set to be sufficiently little. Signals spread with different spread codes appear as broadband noises when despread is performed, and only the target signal can be acquired by selecting a spread code used in despread on a reception side. For this reason, the same frequency band can be used by a plurality of communication channels to perform CDMA (Code Division Multiple Access).
According to the spread spectrum communication system, it is possible not only to perform multiple access with a plurality of transmitters, but also to multiplex a plurality of baseband signals spread with different spread codes on one transmitter for transmission. Such a system for multiplexing a plurality of baseband signals using a plurality of spread codes for transmission is called a multicode system.
FIG. 1 shows a configuration of a conventional transmitter using the spread spectrum communication system of such a multicode system when it is simply formed as an extension of prior arts.
The conventional transmitter using the spread spectrum communication system comprises signal processor 1, baseband signal multiplexing circuit 82, RF (Radio Frequency) transmitter 10, and antenna 11.
Signal processor 1 converts digitized transmission data such as voice and image into transmission frame signals in accordance with the spread spectrum communication system and spreads the transmission data with different spread codes to produce a plurality of baseband signals.
Baseband signal multiplexing circuit 82 comprises baseband filters 301 to 303, adder 701, and D/A converter 9.
Baseband filters 301 to 303 are formed from digital FIR (Finite Impulse Response) filters or the like and limit bands of the respective baseband signals outputted from signal processor 1. Adder 701 adds the baseband signals outputted from respective baseband filters 301 to 303 to produce one baseband signal with code-multiplexing.
D/A converter 9 converts the baseband signal which is a digital signal produced by adder 701 into an analog signal. RF transmitter 10 converts the analog signal converted by D/A converter 9 into a signal at a radio frequency for transmission through antenna 11 as a radio wave.
However, in the aforementioned case where multiple codes are transmitted by multiplexing a plurality of baseband signals spread with different spread codes on one transmitter for transmission, the need arises for adjusting the transmission level of respective codes.
For example, assuming that a signal power for one code transmission is P over the full dynamic range of D/A converter 9 for effective utilization thereof, a signal level for multiple-code transmission of N multiplexed codes is provided at a signal power obtained by N×P when the circuit provides an output with simple addition. As a result, a signal including signal power information exceeds the bit width of D/A converter 9 to cause an overflow. This means that D/A converter 9 should be set to have a wider dynamic range. When N transmission codes are multiplexed with effective utilization of the dynamic range of D/A converter 9 in one code transmission, it is required that the amplitude of an signal applied to D/A converter 9 is adjusted to 1 over the square root of N. The amplitude may be adjusted to 1 over the square root of N to obtain the power of 1/N since the power is proportional to the square of the amplitude.
However, the number of transmission codes is not always constant but changed depending on a type of data to be transmitted (voice, image or the like) or the amount of data. For this reason, adjustment of the amplitude of a signal applied to D/A converter 9 to 1 over the square root of N at all times regardless of the number of multiplexed codes causes a problem that the dynamic range is not fully used and a relative quantization error is increased to deteriorate transmission quality. In this manner, a conventional transmitter using the spread spectrum communication system can not effectively use a dynamic range of a D/A converter when the number of transmission codes is changed.